Perioperative Fluid Requirements
One must pay particular attention to fluid management in younger pediatric patientsbecause these patients have limited margins for error. A programmable infusion pump or a buret with a microdrip chamber is useful for accurate measurements. Drugs can be flushed through low dead-space tubing to minimize unnecessary fluid administration. Fluid overload is diagnosed by prominent veins, flushed skin, increased blood pressure, decreased serum sodium, and a loss of the folds in the upper eyelids.
Fluid therapy can be divided into maintenance, deficit, and replacement requirements.
Maintenance requirements for pediatric patients can be determined by the “4:2:1 rule”: 4 mL/kg/h for the first 10 kg of weight, 2 mL/kg/h for the second 10 kg, and 1 mL/kg/h for each remaining kilogram. The choice of maintenance fluid remains controversial. A solution such as D5½ NS with 20 mEq/L of potas-sium chloride provides adequate dextrose and elec-trolytes at these maintenance infusion rates. D5¼ NS may be a better choice in neonates because of their limited ability to handle sodium loads. Children up to the age of 8 years require 6 mg/kg/min of glucose to maintain euglycemia (40–125 mg/dL); premature neonates require 6–8 mg/kg/min. Older children and adults require only 2 mg/kg/min and in these patients euglycemia is normally well maintained by hepatic glycogenolysis and gluconeogenesis. Both hypoglycemia and hyperglycemia should be avoided; however, the amount of hepatic glucose production is widely variable during major sur-gery and critical illness. Thus glucose infusion rates during longer surgeries, particularly in neonates and infants, should be adjusted based on blood glucose measurements.
In addition to a maintenance infusion, any preoper-ative fluid deficits must be replaced. For example, if a 5-kg infant has not received oral or intravenous fluids for 4 h prior to surgery, a deficit of 80 mL has accrued (5 kg × 4 mL/kg/h × 4 h). In contrast to adults, infants respond to dehydration with decreased blood pres-sure and without increased heart rate. Preoperative fluid deficits are often administered with hourly maintenance requirements in aliquots of 50% in the first hour and 25% in the second and third hours. In the example above, a total of 60 mL would be given in the first hour (80/2 + 20) and 40 mL in the second and third hours (80/4 + 20). Bolus administration of dextrose-containing solutions is avoided to prevent hyperglycemia. Preoperative fluid deficits are usually replaced with a balanced salt solution (eg, lactated Ringer’s injection) or ½NS. In both cases, glucose is omitted to prevent hyperglycemia. Compared with lactated Ringer’s injection, normal saline has the dis-advantage of promoting hyperchloremic acidosis.
Replacement can be subdivided into blood loss and third-space loss.
Blood loss—The blood volume of prematureneonates (100 mL/kg), full-term neonates (85–90 mL/kg), and infants (80 mL/kg) is proportionately larger than that of adults (65–75 mL/kg). An initial hematocrit of 55% in the healthy full-term neonate gradually falls to as low as 30% in the 3-month-old infant before rising to 35% by 6 months. Hemoglobin (Hb) type is also changing during this period: from a 75% concentration of HbF (greater oxygen affin-ity, reduced Pao2, poor tissue unloading) at birth to almost 100% HbA (reduced oxygen affinity, high Pao2, good tissue unloading) by 6 months.
Blood loss has been typically replaced with non–glucose-containing crystalloid (eg, 3 mL of lactated Ringer’s injection for each milliliter of blood lost) or colloid solutions (eg, 1 mL of 5% albumin for each milliliter of blood lost) until the patient’s hematocrit reaches a predetermined lower limit. In recent years there has been increased emphasis on avoiding excessive fluid administra-tion; thus blood loss is now commonly replaced by either colloid (eg, albumin) or packed red cells. In premature and sick neonates, the target hematocrit (for transfusion) may be as great as 40%, whereas in healthy older children a hematocrit of 20–26% is generally well tolerated. Because of their small intravascular volume, neonates and infants are at an increased risk for electrolyte disturbances (eg, hyperglycemia, hyperkalemia, and hypocalce-mia) that can accompany rapid blood transfusion. Dosing of packed red blood cell transfusions. Platelets and fresh frozen plasma, 10–15 mL/kg, should be given when blood loss exceeds 1–2 blood volumes. Recent practice, particularly with blood loss from trauma, favors “earlier” administration of plasma and platelets. One unit of platelets per 10 kg weight raises the platelet count by about 50,000/μL. The pediatric dose of cryoprecipitate is 1 unit/10 kg weight.
2. “Third-space” loss—These losses are impossible to measure and must be estimated by the extent of the surgical procedure. In recent years the third space has even been attributed to overzealous fluid administration during resuscitation.
One popular fluid administration guideline is 0–2 mL/kg/h for relatively atraumatic surgery (eg, strabismus correction where there should be no third-space loss) and up to 6–10 mL/kg/h for trau-matic procedures (eg, abdominal abscess). Third-space loss is usually replaced with lactated Ringer’s injection . It is safe to say that all issues relating to the third space have never been more controversial.